Randall Hulet investigates atoms at temperatures as low as a few nano-Kelvin. At these low temperatures the quantum mechanical wavelengths of the atoms can be as large as 1 micron, greatly altering normal atomic behavior. Hulet and his group have used laser cooling and atom trapping techniques to explore this exotic regime of matter, investigating ultracold atom collisions and quantum statistical effects, such as Bose-Einstein condensation. Collisions between ultracold atoms can reveal the subtle nature of their mutual interaction. Hulet and group use laser spectroscopy to probe these interactions. By this technique they discovered that two 7Li atoms weakly attract each other at very low temperature. Theorists believed that this fact would prevent lithium from undergoing a Bose-Einstein condensation, the paradigm of all quantum statistical phase transitions. In 1995, however, Hulet and his group succeeded in coaxing a gas of magnetically confined 7Li atoms to Bose condense. Subsequent investigation of this novel system has lead to the direct observation of condensate growth and collapse.

Hulet and group later produced the first quantum degenerate Bose/Fermi (7Li / 6Li) mixture of ultracold gases. The size of the fermion atom cloud was stabilized by Fermi pressure, in analogy to a white dwarf or neutron star. More recent investigations have focused on superfluidity of atomic Fermi gases, using Cooper paired 6Li atoms. These pairs are a close analogy to those resulting in superconductivity of some solids. This goal of this work is to better understand the nature of high-temperature superconductors and other strongly correlated materials.

Research Statement - Randall Hulet

The focus of my research program is the study of quantum gases of ultracold atoms. My current activities employ the isotopes of lithium, the fermion 6Li and the boson 7Li, to model complex condensed matter phenomena and materials. Ultracold atomic gases are highly controllable and well-characterized making them ideal systems to test fundamental models of important materials, such as high-temperature superconductors.

In one experiment, Bose-Einstein condensates are used to explore the effect of random disorder on superfluidity, while in another, the fermion counterparts are used to investigate fermionic superfluidity in the strongly-interacting regime. Materials, no matter how pure they may be or how carefully they are prepared, inevitably have some random disorder. This disorder can be caused by crystal defects, impurities, or anything that changes the landscape of how electrons move about in the material. Many years ago, P.W. Anderson suggested that with enough disorder, an electrical conductor can be transformed to an insulator due to localization of the electrons. Disorder can also induce an otherwise superconducting material to become insulating, with implications for important classes of superconductors, such as those made of thin films or small grains. Many of the predicted effects in superconductors, such as those having to do with disorder, are difficult to observe and to quantify because of the complexity of actual materials. Scientists have recently turned to more idealized “stand-in” systems, comprised of gases of ultra-cold atoms, to model real materials. When the atoms are cooled to temperatures below 1 millionth of a degree above absolute zero, they form a Bose-Einstein condensate, which is a superfluid. A superfluid flows without impedance, in analogy to the electrons in a superconductor that flow without electrical resistance. But unlike real materials, the ultra-cold atom stand-ins are so extremely pure and controllable that they can be used as idealized realizations of theoretical models. Disorder can be imposed on a Bose-Einstein condensate by using optical speckle. Speckle is produced by passing a laser beam through a diffusive piece of glass, such as one that has been sand blasted. The variation of the light amplitude across the condensate causes the atoms to experience a random potential, much like the effect of impurities on the electrons in a superconductor. We have used optical speckle to explore the disorder-induced transition from a superfluid to an insulator in a Bose-Einstein condensate of lithium atoms. The strength of the disorder was varied by adjusting the intensity of the laser beam. They found that the flow of the condensate was impeded when the energy scale of the disorder was comparable to the interaction energy of the condensate. Furthermore, by taking images of the atoms while still confined, the Rice group found that only small ripples in the condensate density appear at the onset of localization. It takes much greater disorder strength to completely fragment the condensate into small disconnected pieces. These observations have provided new insight into the nature of the disorder-induced transition that may have implications for the superconductor-insulator transition. In the future, the Rice group plans to reduce the interaction energy of the condensate in an attempt to view the elusive single-particle effect known as Anderson localization.

The pairing of fermions to form composite bosons underlies two of the most remarkable phenomena in physics: superconductivity and superfluidity. Below a certain critical temperature the electrons in a superconductor pair up, one with its spin up and the other with its spin down, forming the supercurrent-carrying Cooper. Although the conventional BCS theory of superconductivity requires an equal number of spin-up and spin-down particles, physicists have long speculated on what happens if this condition is not met. At Rice, we have shown that a mixture of lithium-6 atoms, the fermions, can pair up even when there are unequal numbers of spin-up and spin-down atoms. With equal numbers and at low enough temperature, about 30 nK, the lithium-6 atoms phase separate into a superfluid core surrounded by a normal shell of the majority spin-up atoms. The interface between the superfluid and normal phases is sharp, indicative of a first order phase boundary, and surface tension at this boundary causes a deformation of the superfluid. The confinement potential creates an elongated cigar-like distribution that we observe to become progressively less elongated with increasing number mismatch, as the superfluid is driven to minimize the ratio of the surface area to volume. Phase separation was just one of the scenarios predicted by theorists prior to our experiment. More exotic phases, including one with pairs having non-zero center of mass momentum, have also been predicted. Our current efforts are focused on finding this so-called FFLO phase (after its originators), especially in one dimension where this new phase of matter is predicted to be especially robust. Research with strongly interacting Fermi gases bear on diverse phenomena ranging from high-temperature superconductivity to the dense quark matter predicted to reside in the cores of neutron stars.

Invited speaker. "Detection of Antiferromagnetic Correlations in the Hubbard Model." Canadian Institute for Advanced Research Workshop on Quantum Materials, Banff, Canada. (2/19/14)

Invited speaker. "Finite Range Corrections Near a Feshbach Resonance and their Role in the Efimov Effect." Institute for Nuclear Theory Workshop on Few-Body Universality in Atomic and Nuclear Physics, University of Washington, Seattle. (5/12/14)